The present invention relates to document authentication. The use of electronic documents is gaining popularity and a variety of different formats of electronic documents exist that can be processed by different computer software applications. One example of a common, platform-independent type of electronic document is a PDF (Portable Document Format) document, which has been developed by Adobe Systems Incorporated, San Jose, Calif. PDF documents can be read by PDF readers, such as Adobe® Acrobat® and Adobe® Acrobat® Reader®, or other types of software applications.
While electronic documents are convenient from many points of view, they also present new problems that do not have to be addressed for regular paper documents. One example of such a problem is that an electronic document can be modified in different ways than a conventional printed paper document. Malicious users may, for example, manipulate an electronic document such that the document no longer reflects what the author originally wrote.